As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as an energy source for the mobile devices. Among them is a lithium secondary battery having high energy density and discharge voltage, on which much research has been carried out and which is now commercially and widely used.
Generally, a secondary battery is manufactured by stacking or winding an electrode assembly including a cathode, an anode, and a separator disposed between the cathode and the anode, placing the electrode assembly in a battery case formed of a metal container or a laminate sheet, and injecting an electrolyte into the battery case or impregnating the electrode assembly with the electrolyte.
One of the principal problems to be solved in connection with the secondary battery is to improve the safety of the secondary battery. For example, the secondary battery may explode by high temperature and high pressure which may be induced in the secondary battery due to the abnormal operation of the secondary battery, such as an internal short circuit, overcharge exceeding allowable current and voltage, exposure to high temperature, dropping, or deformation caused by external impact. Particularly for a pouch-shaped secondary battery, the sealing force of a battery case lowers, with the result that an electrolyte may leak from the battery case.
FIG. 1 is an exploded perspective view typically illustrating the general structure of a conventional representative pouch-shaped secondary battery.
Referring to FIG. 1, the pouch-shaped secondary battery 10 includes an electrode assembly 30, pluralities of electrode tabs 40 and 50 extending from the electrode assembly 30, electrode leads 60 and 70 welded to the electrode tabs 40 and 50, respectively, and a battery case 20 for receiving the electrode assembly 30.
The electrode assembly 30 is a power generating element including cathodes and anodes successively stacked while separators are disposed respectively between the cathodes and the anodes. The electrode assembly 30 may be constructed in a stacking structure or a stacking/folding structure. The electrode tabs 40 and 50 extend from corresponding electrode plates of the electrode assembly 30. The electrode leads 60 and 70 are electrically connected to the electrode tabs 40 and 50 extending from the corresponding electrode plates of the electrode assembly 30, respectively, for example, by welding. The electrode leads 60 and 70 are partially exposed to the outside of the battery case 20. To upper and lower surfaces of the electrode leads 60 and 70 are partially attached insulative films 80 for improving sealability between the battery case 20 and the electrode leads 60 and 70 and, at the same time, for securing electrical insulation between the battery case 20 and the electrode leads 60 and 70.
The battery case 20 is formed of an aluminum laminate sheet. The battery case 20 has a space defined therein for receiving the electrode assembly 30. The battery case 20 is formed generally in the shape of a pouch.
The secondary battery 10 is manufactured by thermally welding contact regions at the outer circumference of the battery case 20 while the electrode assembly 30 is mounted in the receiving space of the battery case 20.
When the secondary battery is put in an abnormal operating condition, such as an internal short circuit, overcharge, or exposure to high temperature, an electrolyte in the secondary battery is decomposed, with the result that high-pressure gas is generated. The generated high-pressure gas may deform the battery case and shorten the life span of the secondary battery. Furthermore, the secondary battery may catch fire or explode due to the high-pressure gas. Therefore, it is preferred to separate thermally welded regions from each other, such that gas is exhausted outside through the separated regions, before the pressure of the secondary battery reaches a high-pressure level at which the secondary battery may catch fire or explode. However, when gas noxious to a human body is exhausted outside through an arbitrary region, it is difficult to control the exhaust of the noxious gas.
Therefore, various attempts have been made to prevent the combustion or explosion of a battery, when high-pressure gas is generated, and efficiently exhaust the gas outside.
As an example, Japanese Patent Application Publication No. 2006-185713 discloses a secondary battery constructed in a structure in which upper and lower parts, of which the inner surfaces are made of a flexible, thermoplastic resin, of a battery case are overlapped, and flat portions around a power generating element receiving part are thermally welded, wherein a vertically bent concavo-convex part is partially formed in the thermally welded region.
In the above-described technology, the vertically bent concavo-convex part serves to selectively exhaust high-pressure gas generated from the interior of the battery case through the corresponding region. However, the disclosed secondary battery has a problem in that the thermally welded flat portions around the vertically bent concavo-convex part are minutely deformed by the concavo-convex part, with the result that a sealing force of the secondary battery decreases.
As another example, Japanese Patent Application Publication No. 2005-116235 discloses a technology for forming a specific gas exhaust mechanism in a battery constructed in a structure in which the outer circumferential portions of a laminate film, in which an electrode assembly is mounted, are thermally welded to seal the laminate film. The disclosed technology is to deform the innermost sealing layer of the laminate film such that the innermost sealing layer has an adhesive strength by thermal welding less than that of the remaining region of the laminate film using a method of partially removing a sealing layer or coupling a polymer resin exhibiting a low coupling force to some of the sealing layer.
In the method of partially removing the sealing layer, however, the sealing layer region and the region around the sealing layer are relatively weak. As a result, internal gas may be concentrated on these regions, even under the pressure generated in a state in which the battery normally operates, and therefore, the regions may be easily ruptured. Consequently, it is difficult for the sealing layer to secure sealability. In the method of coupling the polymer resin exhibiting the low coupling force to some of the sealing layer, on the other hand, sealability at the interface between the different polymers may greatly lower due to the difference in material between the polymer resin of the sealing layer and the polymer resin exhibiting the low coupling force.
Although the above-described technologies may exhaust high-pressure gas through the selected region to secure safety, therefore, it is difficult to exhibit reliable sealability in a state in which the battery normally operates.